1. Field of the Invention
The invention relates to pulp production and somewhat more particularly to the process of preparing pulp from a lignocellulosic material, such as coniferous or deciduous wood.
2. Prior Art
Production of pulp from lignocellulosic materials is well known and may involve mechanical, chemical, and thermal processes, or a select combination of such processes to produce cellulosic fibers which can be manufactured into various products, for example, paper. Particularly economically attractive processes typically involve chemical pulping, semichemical pulping and/or chemi-thermomechanical pulping, due to relatively high pulp yields.
Typically, in chemical pulping processes, shredded or chipped lignocellulosic materials are subjected to chemical reagents that remove at least partially, as by dissolving, extracting, dispersing or the like, lignin, hemicellulose, gums, carbohydrates, fatty materials, etc., collectively referred to as "resins" from, for example, wood chips to release cellulose fibers during a digestion process. A presently dominant chemical pulping process in the U.S. and certain other regions of the world is a so-called Kraft process. In the Kraft or sulphate process, sodium hydroxide and sodium sulfite typically comprise the principal cooking or digestive chemicals, which when admixed with water are generally referred to as alkaline pulping liquor or white liquor. The alkaline reagents react with lignin and other resin molecules, breaking them into smaller segments whose sodium salts are soluble or dispersable in the cooking liquor.
In the Kraft pulping process, a select amount of, for example wood chips, which may be pre-treated with steam or water that may also include chemical reagents, are charged to a digester vessel, along with alkaline pulping liquor to attain a select chemical or liquid-to-wood ratio and this material charge is then subjected to controlled heat and pressure over a select period of time. Both batch and continuous digestion processes are known. In batch processes, the material charged may be held under select temperature/pressure condition for a calculated period of time to attain a desired pulp characteristic and then discharged or "blown" into a holding tank so as to yield a pre-calculated amount of pulp suitable for further processing, such as chemical and/or heat recovery, washing, further digestive-type processing, bleaching, etc. prior to, for example, paper manufacturing. In a continuous digestion process, the material charge is controllably moved through zones of select temperature/pressure to a regulated discharge point, (i.e., a valve) to continuously yield pulp having desired characteristics (i.e., reduced "resins" content, a select Kappa number or range, water drainability, etc.).
A primary object of a pulping or digestion process is to reduce the amount of "resins" present in pulp fibers without deleteriously affecting paper forming characteristics while maintaining an economically viable process and product (pulp) costs. An emphasis has been placed in pulping processes involving chemical means to provide chemical reagents compatible with the digestion conditions and cooking chemicals and which aid deresination. For example, U.S. Pat. No. 2,716,058 describes the use of ethoxylated phenols and tall oil as deresination agents. U.S. Pat. No. 2,999,045, on the other hand, describes the use of copolymers of polyethylenoxide and polypropylenoxide as deresination agents. Organic solvents, such as kerosene, methanol, etc. and various emulsifiers such as certain sulfonated fatty acids (see U.S. Pat. No. 4,673,400) and solubilizint agents, such as C.sub.12 alpha-olefin sulfonates (see U.S. Pat. No. 4,426,254) have also been suggested as deresination agents. In addition, anthraquinione and certain derivatives thereof have been suggested as additives or catalysts useful in cooking liquors for deresination of lignocellulosic materials, see U.S. Pat. No. 4,012,280. However, these various additives, solvents, catalysts, etc. tend to exhibit various drawbacks, including high chemical costs, excessive processing time, incompatibility with typical alkaline processing parameters, etc.
It is therefore an object of the invention to provide an improved method of producing pulps, such as paperboard pulps, containerboard pulps, linerboard pulps, corrugated medium pulps, Kraft or sulphate market pulps, etc., by adding a digestion additive to alkaline pulping liquor in a pulping process to attain a reduced H-factor, reduced material rejects, reduced "fresh" pulping liquor requirements and reduced cooking or digestion time periods relative to heretofore practiced pulping processes.
Other objects, features and advantages of this invention will become more apparent from the detailed description that follows.